This invention relates to glazing intended for automotive roofs comprising a part providing a desired interior luminous transmittance.
Automotive manufacturers are developing models having increasing glazed surface areas. The dimensions of windscreens and rear screens are increasing particularly to improve aerodynamic profiles. In addition, glazings are becoming preponderant in the manufacture of sunroofs. Following this tendency, manufacturers wish to use glazings to provide the whole of, or at least a significant part of the roofs of vehicles.
The aim of this invention is to provide glazings for vehicle roofs, having at least one portion having a desired transparency, and at least another portion which must satisfy different requirements of transparency. A typical case of glazings satisfying this requirement is that of glazings provided with photovoltaic cells. The invention is described in relation to glazings adapted to comprise cells of this type. However, the invention may be applied to other sort of glazings in respect of which only a part must offer the indicated transparency.
On certain models, particularly top of the range models, automotive manufacturers propose photovoltaic cells whose function is to limit the drain on the batteries, particularly when the vehicle motor is not running. This is currently used for example to recharge the batteries or to power a ventilation system and thus limit heating of the vehicle when stopped. Other applications have already been envisaged all of which have in common the provision of a complementary energy source to reduce the use of energy generated by the vehicle motor.
For good efficiency, the cells may be arranged on a surface which is as large as possible, and underneath a shield which protects them from environmental elements: humidity, grease etc. The cells are thus placed behind a transparent screen, usually provided by a sheet of glass.
For convenience, the cells have generally been arranged at the roof in vehicles which have already been commercialised. The adaptation of photovoltaic cells on glass roof elements has been the subject of various previous propositions.
With respect to these types of commercialised vehicles incorporating this type of equipment, the photovoltaic cells have been arranged on a sheet of glass forming at least part of the roof, and on the face of the sheet which is not exposed to the exterior. The cells are adhered to the sheet of glass and protected and hidden from the inside of the vehicle by a facade.
One difficulty for the manufacture of these roofs comprising cells resides in the fact that in order to satisfy safety standards, particularly with respect to mechanical resistance, the glass sheet must have a certain thickness. The thicker the glass, the less the energy transmitted to the cells.
In known articles, a sheet of tempered glass at least 5 mm thick has typically been necessary. At these thicknesses, ordinary clear glass has an absorption which is not negligible. The energy transmittance (ET) measured in the Moon system is about 82% of the incident radiation.
These known articles lead to a non-optimised efficiency for the cells. It is desirable to find a construction which reduces the absorption to a minimum without compromising the mechanical properties.
In this context, it is possible to use so-called xe2x80x9cextra clearxe2x80x9d glasses whilst keeping the structure described above. Theses glasses which have the particularity of having an extremely low quantity of iron have relatively low levels of absorption. At thicknesses under 5 mm, as previously described, the ET can be of the order of 90%, thus giving a gain of the order of 8%. The disadvantage of this glasses is its cost. The cost is approximately 2.5 times that of ordinary clear glass. Even if the cost of the glass only contributes to part of the total cost of the roof, this difference is not insignificant for the manufacturers.
Furthermore, in known structures, the use of a sheet of glass having an energy transmittance as great as possible to increase the efficiency of the photovoltaic cells is directly contradictory to the requirements of motor vehicle manufacturers regarding the transmittance in the interior of the vehicle. For reasons of comfort, the energy transmittance should be as low as possible, should not be greater than 20%, and should preferably be less than 15%. The use of clear glass and particularly extra clear glass, can only be envisaged if only a part of the roof will receive the cells, the rest being adapted for the vision of the passengers.
In order to satisfy the contradictory requirements previously referred to, it is possible to deposit a thin coating using a traditional techniques to limit the transmittance. The coating must nevertheless be limited to part of the sheet which does not carry cells, which increases in the complexity of the operation. In addition, the coating is relatively fragile, irrespective of the technique to form it, and at the interior of the vehicle, it remains exposed to the risk of degradation, abrasion etc. Faults resulting from such degradation are extremely sensitive to the extent that the coating absorbs and/or reflects a significant part of the incident light. Faults in the coating thus appear as luminous points or lines on a surface which is significantly less luminous.
The invention provides glazings for vehicle roofs having a new structure which responds to the previously described requirements. The glazings according to invention comprise at least one transparent portion, and at least one portion which is not transparent, particularly due to the presence of photovoltaic cells or other functional elements. This solution offers a number of advantages which will be described in more detail.
The following description refers to a vehicle roof. This designation relates to the case where the whole of the roof is envisaged. It also relates to the case where only part of the roof is equipped in the way described. It can easily be understood that the xe2x80x9cglass portionxe2x80x9d may be limited for example to the part comprising the movable element of a sunroof. The case of the complete roof corresponds best when seeking to arrange the cells on as large an area as possible, without necessarily ignoring other aspects. In addition the tendency in automotive design, referred to above, is clearly towards an increase in the glazed surface area. Embodiments according to invention which follow this tendency favour a roof made entirely of glass. The description and the examples referred in a non-limiting way to a roof made entirely of glass.
The roof glazings according to the present invention provide at least one transparent portion and one portion which is not transparent due to the presence of non transparent functional elements. It comprises a first sheet of glass comprising the external surface, a second sheet of glass comprising at least part of the internal surface of the roof, and an intermediate sheet of one or more thermoplastic materials traditionally used for forming laminated glazings, the intermediate sheet extending at least over the portions of the sheets of glass which are face-to-face. The non transparent functional elements, for example photovoltaic cells, are arranged under the first sheet of glass.
In the following, for reasons of convenience, photovoltaic cells are referred to in terms of functional elements. This example is particularly representative. However, the elements may be of any other non-transparent type which cover a significant portion of the surface of the roof.
The combined characteristics of the elements superposed in the transparent portion of the glazings are such that the energy transmittance of this portion is not greater than 25%, and preferably remains less than 20%. In the transparent portion, the luminous transmittance is at most 25%, and is usually less than 30%.
When the functional elements are photovoltaic cells, the first sheet of glass is chosen such that its energy transmittance ET is at least 82% and is preferably greater than 87%.
The laminated structure may extend over the whole of the glazings or only over a portion of the glazings. The transparent portion of the roof is always laminated. In contrast, the non transparent portion of the roof, the part where the cells are arranged, may comprise only the external sheet of glass.
In the case of a glazing which is partially laminated, it is advantageous to arrange for the first sheet of glass, which is the sheet of glass having the greatest dimensions, to be the only one that rests on the bodywork. It is important for reasons of assembly and sealing to avoid differences of level. For this reason, the choice of the dimensions of the sheet may lead to a first sheet having a periphery which is free over a few millimeters. In other words, the second sheet of glass is always set back a distance at least equal to that necessary for the positioning of the glazings on the bodywork.
By an appropriate choice of the sheet of glass and, or of the other constituents of the laminated assembly: coatings, enamel, properties of the intermediate sheet, it is possible to satisfy the requirements for energy transmittance in the vehicle. These requirements are directly contradictory to those regarding the exposure of the cells. In other words, the laminated structure allows the distinct and opposed requirements to be met according to the part of the glazings considered.
Roof glazings according to the invention also meets the mechanical resistance requirements of the designers. The roof contributes to the rigidity of the overall structure. Whichever form is chosen, wholly or partially laminated, the glazings according to invention provide the required mechanical properties under conditions indicated below.
If only it portion is laminated and transparent, the other portion which carries the cells being non transparent, the mechanical properties may be obtained in part by structural reinforcing elements such as metallic profiles or sheets arranged under the non laminated portion of the glazing. These elements which are integrated into the vehicle bodywork do not provide an interference given that they are positioned under the non transparent portion of the roof. These additional structural elements, and also the cells and various other elements, are masked from the interior of the vehicle by traditional coverings and facades.
Use of a glazing which is partially laminated instead of a glazing which is entirely laminated also allows for a significant weight gain. This aspect is of more significance given that the second sheet of glass, which is of restricted dimensions, is that which usually has the greatest thickness for the reasons described below particularly relating to energy transmittance.
When the glazing is entirely laminated, the inherent mechanical resistance of this type of structure may be sufficient to attain the required performance. In this case it is not necessary to arrange additional reinforcing elements under the non transparent portion of the glazing.
In embodiments according to invention, the sheets are assembled according to the usual techniques for obtaining laminated glazings.
In comparison with prior art constructions, the laminated glazing assembly allows, in particular reduction in the thickness of the sheet of glass which protects the cells when the lamination is carried out on the entire glazing and the cells are arranged between the two sheet of glass. A significant part of the resistance of the assembly is thus conferred by the second sheet glass whose energy transmittance characteristics, and thus thickness, are not determined by the presence of the photovoltaic cells.
Whilst the first sheet is advantageously of limited thickness, the total thickness of the laminated assembly influences the mechanical properties. As in the case of monolithic glazings, considerations of weight lead to a limiting of this thickness. It is best to try to keep the total thickness less than 10 millimeters, and preferably less than 7 mm.
As indicated above, according to whether the laminated assembly extends over practically the entire glazing surface or is limited to the transparent portion of roof, the thickness of the first sheet of glass is significantly different. However, in both cases, the first sheet of glass contributes to the mechanical resistance of the assembly. In the first case, the exterior sheet of glass must have a suitable thickness to provide suitable resistance to external forces, independently of the question of rigidifying the vehicle compartment. In the second case, the contribution of the first sheet of glass to this rigidifying is more significant.
For these reasons, in the first case sheets having a thickness which is not less than 1 mm are advantageously used. Conversely, in order to conserve the energy advantage even with ordinary soda lime glass, the thickness of this first sheet is not greater than 3 mm. In most cases, this thickness is comprised between 1.5 and 2.5 mm. For xe2x80x9cclearxe2x80x9d and xe2x80x9cextra clearxe2x80x9d glass, the thickness of the first sheet may be greater without losing the benefit of a higher level of energy transmittance. For these clear glasses, the thickness may reach 5 mm.
In the second casexe2x80x94that of xe2x80x9cpartial laminationxe2x80x9dxe2x80x94the thickness depends on whether reinforcing elements for the structure are present or not. This thickness is necessarily greater than in the first case, and is advantageously from 2 to 6 mm, preferably from 3 to 5 mm.
The first sheet may, of course, be an extra clear glass to optimize the transmittance. In this case, the energy transmittance of the sheet may be greater than 90%. If a sheet of clear glass is used, the energy transmittance, again with respect to the thickness conditions referred to previously, is a little less but remains greater than 85%. By way of example, the energy transmittance of a commercially available extra clear glass at a thickness of 4 mm is 90.7. For a commercially available clear glass, again at 4 mm, the energy transmittance is 89.5. Of course, the thinner the sheet the higher the transmittance.
The second sheet of glass is chosen so as to provide the necessary resistance at least in the transparent portion. Considerations regarding its thickness with respect to the luminous transmittance are the opposite to those concerning the first sheet. In order for the roof to allow vision towards the outside whilst limiting energy transmittance in the interior, the second sheet must be very absorbent, and thickness is a significant factor in providing this absorption. In practice however a compromise must be chosen between the increase in this thickness, which is favorable for the mechanical resistance and for the absorption on one hand, and the necessity to keep the weight within reasonable limits on the other hand.
From the mechanical point of view, in the laminated assembly, and in association with the first sheets described above, sheets of 2 to 5 mm and, preferably, of 2.5 to 4 mm allow the standards in this field to be met. For these thicknesses, to obtain an energy transmittance not greater than 25%, and preferably not greater than 20 percent, strongly colored glasses are particularly used.
Due to the significance of the role of the second sheet of glass in glazing embodiments according to the invention, its nature will be described in more detail.
Coloured glasses which are useful in accordance with the invention are known from the prior art. Amongst glasses which allow a significant reduction in the energy transmittance, those having a neutral, a blueish, or a blue green colour in transmittance are preferred. In any case manufacturers desire a colour purity, in the sense of the CIE (Commission International de L""Eclairage) which is as low as possible. The aim of choosing these glasses is so that the light transmitted to the interior does not the deform the colours. Advantageously, a sheet of gray glass having an excitation purity less than 10% is used, and which at a thickness of 4 mm has a luminous transmittance (LTA) of less than 25%, and preferably less than 20%. Glasses corresponding to these conditions may be, for example, soda lime glasses having, in a traditional way, base constituents in the following relative weights:
Colouring agents, particularly Fe2 O3, Co, Se, Cr2 O3, are added. xe2x80x9cGreyxe2x80x9d glasses of this type are particularly those having colouring agents in the following proportions:
Another advantageous combination of colouring agents comprises chromium oxide. Preferred proportions are for example:
Glasses of this type are described in detail particularly in the publications FR-A-2 738 238 and 2 738 240.
All of the preceding glasses are very neutral and xe2x80x9cgrayxe2x80x9d in transmittance. In other cases, as previously indicated, glazings in accordance with invention may have a blueish tint. To provide this type of glazing it is advantageous to use for the second sheet of glass essentially oxides of iron and cobalt as colouring agents in the following proportions:
to which other agents may be added in the following limits:
Blue glasses satisfying this definition are described in detail in the European patent application filed on Dec. 22, 1998 under the number 98124371.0.
It is also possible to use a glass having a high selectivity (the ratio LTA/ET) such as those having colorants in the proportions:
Theses glasses have a very dark colour, with a green to blue tint. Their selectivity is often greater than 1.65. They are described in detail in the French patent application filed on July 31 under the number 98/10020.
Another series of very selective coloured glasses having low energy transmittance which may be used for the second sheet of the laminated portion of the roof corresponds to compositions in which the colorants are: either
These glasses which are also highly coloured are gray green. They have a selectivity which is normally greater than 1.5. They are described in the publication EP-A-0887320.
The colorimetric characteristics of the second sheet of glass preferably satisfying the following relations:
P less than 20
R less than xe2x88x92P+80
in which P is the excitation purity (CIE) measured at a thickness of 4 mm with illuminant C at 2xc2x0 observer and R is the colour rendering index specified in the standard EN 410. This latter index relates the observations through a particular glazing to an assembly of the sample colours illuminated by reference illuminant D65. The less than the glazing modifies the perception of colours, the higher the colour rendering index. The proposed gray glasses are those having the highest colour rendering index. It is generally greater than 80% and can attain and even be greater than 90%. In comparison, the glasses which confer a blueish tint generally have a lower index of about 75%. Generally, sheets having a colour rendering index not less than 70 and preferably not less than 75% are used in glazings according to the invention.
The most neutral glasses, which are gray in colour, preferably satisfy the conditions:
xe2x80x83P less than 10
R less than xe2x88x92P+90
The use of highly coloured glasses usually leads to the desired transmittance. If however the chosen glass does not sufficiently reduce the transmittance, or if it is preferred to use a glass which is not as highly coloured, it is possible to provide the desired transmittance properties by use of a traditional thin absorbent and/or reflective coating, for example a coating based on titanium or chromium nitride, a coating of tin oxide which may be doped, a coating of indium and tin oxides etc. When such a coating is used it is advantageously protected from the risks of degradation by being arranged on the face of the sheet of glass which is in contact with the intermediate layer.
To enable a coating which uniformly covers the sheet to be provided, it is preferably arranged on the second sheet of glass. This avoids the necessity of accurately limiting the extent of the coating with respect to the position of the cells. Conversely, if the coating is applied on the first sheet, the position corresponding to the cells must be free of the coating to provide the greatest possible transmittance at these positions.
Another means of reducing the luminous transmittance which may be used according to the present invention consists of depositing an enamel comprising small sized points in a dense grid. The points must be of a sufficiently small size such that they are not discernible by observation from the interior. They are below the resolution threshold. Points of the few tenths of millimeters spaced about 0.5 to 2 millimeters apart may be chosen.
With an enamel pattern limiting the transmittance, the proportion of the surface covered determines the proportion of the non transmitted radiation. This proportion may be varied to a very large extent. The enamel may cover up to 70% of the surface whilst maintaining a certain xe2x80x9ctransparencyxe2x80x9d. Preferably, it does not cover more than 60% of the surface.
When a pattern of enamel dots is used, it may be provided on one or the other of the sheets of glass provided that this pattern does not mask the cells.
An intermediate sheet of a thermoplastic material is arranged between the two sheets of glass. During lamination there may be a plurality of sheets of the same or of different materials making up the intermediate sheet. The thermoplastic sheet may also be formed in the assembly from a different state, particularly by polymerization or reticulation of a material in a liquid state. The indication intermediate sheet thus corresponds to the final form in the assembly and does not relate to the initial state of material even if the most usual form is that of a sheet.
The intermediate sheets are those generally used in the laminated glazings. In particular, sheets of polyvinyl butyral (PVB), polyvinyl acetate (EVA), polyurethane (PU) and polyvinyl chloride (PVC) may be used. When the cells are arranged between two sheets of glass, the thickness of the intermediate sheet or sheets must, be at least equal to that of the photovoltaic cells arranged in the assembly. In practice, the cells have a thickness between 0.1 and 1 mm. The intermediate sheets of ordinary laminated glazings have a thickness of the order of 0.3 to 2 millimeters. These thicknesses are thus suitable whether the cells are arranged in the lamination or not.
The absorption of the intermediate sheets is generally sufficiently low so as only to have a limited effect on the transmittance of the assembly. When a plurality of sheets is use to make up the intermediate sheet, it is nevertheless possible to combine their characteristics to respond to the aims sought by the invention. In particular, in the case of cells arranged between the two sheets of glass, it is possible to arrange a sheet of a material which contributes to the luminous absorption underneath the cells without detracting from the radiation received by the cells. In particular, materials such as coloured PVB in respect of which the energy transmittance for typical thicknesses of 0.76 mm may be as low as 15% may be used. Of course, commercial products allow a range of transmittances of intermediate values to be achieved.
When cells are arranged in the non laminated portion of the glazing, they may be arranged by adhesion to the first sheet of glass. Various adhesives may be used provided that they do not block the energy transmittance to the cells. The cells may also be secured by means of the thermoplastic sheet used for lamination. In this case, the thermoplastic sheet extends beyond the laminated portion to additionally cover the zone of the cells. A sheet of EVA is particularly well suited to this type of construction. In another embodiment, the cells may be secured by an adhesive of sheet arranged so as to envelop them.
When cells are arranged in the lamination, the method of introducing them in the intermediate sheet depends in part on the malleability of the intermediate sheet. For products which are easily formed, it is possible to print the impression corresponding to the cells in the sheet and to subsequently position the cell in the said impression. For sheets which are less easily formed, it may be preferable to associate at least two sheets of which one has a thickness which is substantially equal to that of the cells. The sheet is stamped to cut out housings to the dimensions of the cells. It is subsequently associated with at least one sheet to constitute an assembly similar to the printed sheet described above. In both cases, an additional sheet is superpositioned when necessary with the assembly carrying the cells and the electrical connections so as to entirely cover the cells in a relatively flexible product and avoid contact with the surfaces of the sheets of glass.
In order to facilitate incorporation of the cells in the intermediate sheet, characteristics of different materials may be combined. For example, a preformed sheet of PVB comprising housings for the cells may be associated with a flexible film, for example of EVA, which allows a perfectly uniform introduction and adhesion of the cells in the intermediate sheet.
For aesthetic reasons, it is also advantageous according to the invention to mask the edges of the cells or at least those comprising electrical connections, as well as the connections, and generally all parts which introduce a discontinuity in the appearance on the product, by means of an enamel in a pattern obtained by silk screen printing. When these enamel portions are intended to mask discontinuities from the outside, they are arranged on the first sheet of glass.
Glazings according to the invention are laminated using traditional techniques. For preformed intermediate sheets, such as PVB sheets, two stages are generally followed: a first step of de-gassing followed by an adhesion step. The technique used is particularly described in the publication FR-A-2428920, relative to in encapsulation of photovoltaic cells in a laminated assembly.
Other methods of lamination may also be used, particularly when a liquid material is used to constitute the intermediate sheet.